Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates: I heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever shares ownership of a slave with another person and frees his own share, a value shall be set against him (for the remaining share), and then he shall free him (completely)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13583